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printk: nbcon: Allow KDB to acquire the NBCON context

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KDB can interrupt any console to execute the "mirrored printing" at any
time, so add an exception to nbcon_context_try_acquire_direct to allow
to get the context if the current CPU is the same as kdb_printf_cpu.

This change will be necessary for the next patch, which fixes
kdb_msg_write to work with NBCON consoles by calling ->write_atomic on
such consoles. But to print it first needs to acquire the ownership of
the console, so nbcon_context_try_acquire_direct is fixed here.

Reviewed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://patch.msgid.link/20251016-nbcon-kgdboc-v6-3-866aac60a80e@suse.com
[pmladek@suse.com: Fix compilation with !CONFIG_KGDB_KDB.]
Signed-off-by: Petr Mladek <pmladek@suse.com>
This commit is contained in:
Marcos Paulo de Souza 2025-10-16 11:47:56 -03:00 committed by Petr Mladek
parent 49f7d3054e
commit 286b113d70
2 changed files with 21 additions and 1 deletions
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16
include/linux/kdb.h
View File

@ -14,6 +14,7 @@
*/
#include <linux/list.h>
#include <linux/smp.h>
/* Shifted versions of the command enable bits are be used if the command
* has no arguments (see kdb_check_flags). This allows commands, such as
@ -207,11 +208,26 @@ static inline const char *kdb_walk_kallsyms(loff_t *pos)
/* Dynamic kdb shell command registration */
extern int kdb_register(kdbtab_t *cmd);
extern void kdb_unregister(kdbtab_t *cmd);
/* Return true when KDB as locked for printing a message on this CPU. */
static inline
bool kdb_printf_on_this_cpu(void)
{
/*
* We can use raw_smp_processor_id() here because the task could
* not get migrated when KDB has locked for printing on this CPU.
*/
return unlikely(READ_ONCE(kdb_printf_cpu) == raw_smp_processor_id());
}
#else /* ! CONFIG_KGDB_KDB */
static inline __printf(1, 2) int kdb_printf(const char *fmt, ...) { return 0; }
static inline void kdb_init(int level) {}
static inline int kdb_register(kdbtab_t *cmd) { return 0; }
static inline void kdb_unregister(kdbtab_t *cmd) {}
static inline bool kdb_printf_on_this_cpu(void) { return false; }
#endif /* CONFIG_KGDB_KDB */
enum {
KDB_NOT_INITIALIZED,

6
kernel/printk/nbcon.c
View File

@ -10,6 +10,7 @@
#include <linux/export.h>
#include <linux/init.h>
#include <linux/irqflags.h>
#include <linux/kdb.h>
#include <linux/kthread.h>
#include <linux/minmax.h>
#include <linux/panic.h>
@ -249,13 +250,16 @@ static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt,
* since all non-panic CPUs are stopped during panic(), it
* is safer to have them avoid gaining console ownership.
*
* If this acquire is a reacquire (and an unsafe takeover
* One exception is when kdb has locked for printing on this CPU.
*
* Second exception is a reacquire (and an unsafe takeover
* has not previously occurred) then it is allowed to attempt
* a direct acquire in panic. This gives console drivers an
* opportunity to perform any necessary cleanup if they were
* interrupted by the panic CPU while printing.
*/
if (panic_on_other_cpu() &&
!kdb_printf_on_this_cpu() &&
(!is_reacquire || cur->unsafe_takeover)) {
return -EPERM;
}